Strain-rate-dependent behaviors of CoCrNiSi0.2 medium-entropy alloy: Deformation mechanism and microstructural-based constitutive modeling

Abstract

Abundant strengthening mechanisms in medium-entropy alloys (MEAs) endow them with excellent comprehensive properties, rendering them promising candidates for use in extreme service conditions, such as low/high temperatures and strong impact environment. Investigation of the mechanical behaviors and micro deformation mechanisms of MEAs under intermediate/high strain rate loadings should therefore help to explore their application potential in impact-resistant structures. In this study, the tensile mechanical properties of a single-phase face-centered cubic (FCC) CoCrNiSi0.2 MEA were tested over low, intermediate, and high strain rates, and the micro deformation mechanisms at different strain rates were analyzed systematically. Dislocation evolution, deformation twinning, and phase transformation (from single FCC to FCC + hexagonal close-packed (HCP)) were observed in the three groups of tensile specimens loaded at low, intermediate, and high strain rates. The dislocation densities and twin volume fractions in the three groups were comparable, while the volume fraction of phase transformation increased significantly with the increasing strain rate. Therefore, more HCP phases generated during high-strain-rate deformation should lead to a stronger precipitation-strengthening effect on the material, which made the strain-rate-strengthening effect more significant to the extent that it could not be ignored in constitutive modeling. Finally, the contribution of the phase transformation was introduced for the first time into a constitutive model in which dislocation slip and deformation twinning were also considered to describe the viscoplastic flow behavior of the CoCrNiSi0.2 MEA over a wide range of strain rates.